Various tradeoffs are made in designing high fidelity sound equipment. Considerations involved in the design decision include size, power output, distortion, loudspeaker protection, cost, freedom from clipping, efficiency, ability to drive low impedance, heat dissipation, large damping factor, use of low cost parts and reliability. Some factors are often sacrificed for others. Thus, low cost may be of primary importance, or having a large damping factor may be of greater significance.
Apparent but unnecessary conflicts may exist between these factors. For example, an amplifier with a large damping factor will have a large loop gain. This can cause the amplifier to sound "bad" when it clips as the loop gain tends to over-drive the output stages when driven into clipping (saturation). This is the main reason why some decide that tube amplifiers or amplifiers with low loop gain sound better. But it is possible to design an amplifier with much loop gain without significant distortion from clipping.
Clipping tends to be the most serious form of distortion in an amplifier, as it is most unnatural. Harmonic distortion is made up of frequencies which are multiples of the main frequency. For this reason, they are similar to the frequencies in music. IM (inharmonic distortion) is composed of signals which are the addition and difference of two input frequencies. This form of distortion is not as acceptable as harmonic distortion, but less serious than clipping.
Clipping produces frequencies which have no relationship to the input frequency. The percentage of distortion can go very quickly above 10% once clipping starts. Clipping occurs when the input requests that the output signal from the amplifier be too large for it to reproduce. The internal circuitry of the amplifier continues to try to answer this request and therefore drives the output transistors into saturation. Once the output transistors are in saturation, they will stay in that condition long after the input signal is removed. If the amplifier has much internal gain, the problem can be worse as the internal circuitry will try even that much harder to track the input.
An amplifier goes into saturation when it is asked to drive a load with a voltage too "close" to its power supply voltage rails. For music, an amplifier is generally asked to do this quite often. Various studies have shown that the ratio of peak voltages in music to RMS is in the range of 17 to 20 db in high quality material. Assuming a figure of 17 db. (which is a factor of 50 to 1) for an amplifier to deliver 2 watts RMS, it must be able to handle peak outputs of over 100 watts. This explains why in order to have good fidelity, many amplifiers are in the range of 100 to 200 watts. They are in fact simply trying to reproduce 2 to 5 watts cleanly.
The ratio of peak power to RMS power is commonly referred to as headroom. Whenever an amplifier has less headroom than the music it is reproducing requires, there will be clipping. This clipping can be either soft, hard or uncontrolled. In the past, anti-clipping circuits have been used to prevent clipping distortion. Soft anticlipping circuits, i.e. those circuits which sense the input to reduce gain have been used, but some clipping can still occur and this type of circuit reduces the dynamic range. Hard anti-clipping circuits reduce the drive when needed to prevent clipping.
Here, soft clipping will be defined as where there is a circuit which monitors the output (or input) and reduces the gain of the amplifier to a point where it is expected not to clip to a significant extent. Hard clipping is defined as an amplifier with a circuit that prevents output transistors from being driven into saturation and therefore, the amplifier does not exhibit the undesirable persistence side effects of clipping, i.e. the output transistors staying in saturation after the input signal has gone away. Uncontrolled saturation as exists in "normal" circuits where the output transistor can be over-driven by the driving circuits into saturation which persists long after the loss of the input signal.
A commonly designed amplifier rated 100 watts per channel with a 3 db headroom will deliver 200 watts peak power. At an output of 4 watts RMS it has a headroom of 17 db. Above this power, clipping will start to occur more often. On high quality music, this amplifier will not be used above this level of 4 watts per channel RMS.
The problem is that the above 100 watt amplifier will generally have a very large power supply, possibly even a regulated power supply. It will have large output transistors with large heat sinks and most probably will have output protection circuitry on the amplifier. Yet, this is all to reproduce only 4 watts RMS per channel.